As one of copyright protecting techniques in a DVD (Digital Versatile Disc)-ROM (Read Only Memory), a recording type DVD, or the like, there is such a technique that in a state of an un-recorded disc, binary information which is called a media ID and is peculiar to the medium is recorded into an innermost rim area (Burst Cutting Area; hereinbelow, referred to as BCA) and contents data which is recorded is encrypted by using the media ID.
Also in a high density optical disc such as a BLT-ray disc (registered trademark; hereinbelow, referred to as BD) or the like, as disclosed in JP-A-2005-518055, such a technique that a bar-code-like mark (hereinbelow, referred to as a BCA mark) as binary information is recorded into the BCA has been proposed.
In JP-A-2005-518055, the following techniques have been disclosed: the BCA mark is recorded by using a crystalline state and an amorphous state of a phase change recording film; grooves or pits are formed in the BCA at a track pitch which is five or more times as large as a track pitch of adjacent areas; the BCA mark is formed in the radial direction so as to transverse a plurality of tracks; and the like. Further, the following technique has been disclosed: in the case of reproducing the BCA mark, a reproduction waveform obtained when the BCA mark is reproduced by using the grooves by tracking servo control is compared with a reproduction waveform obtained when the BCA mark is reproduced without making the tracking servo control, thereby detecting alteration of the BCA mark.
In a recording area where a signal is recorded onto an ordinary track, the signal is recorded or reproduced in a state where a spot of a laser beam has been focused onto a signal surface by focusing servo control and the track is accurately being traced by the tracking servo control. On the other hand, the BCA is an area where a consideration is taken to a technique for enabling information to be more easily read out in a state where an in-focus state according to the focusing servo control is merely obtained without making the tracking servo control.
It is assumed that the grooves (that is, tracks) have been formed also in the BCA and a signal recording layer made by an alloy, a layer separating type, or the like is also continuously formed from, for example, a lead-in area. In the BCA which is formed so as to have the signal recording layer as mentioned above, the BCA mark can be formed as follows.
For example, at a manufacturing stage, in an initial state, the signal recording layer is in an un-recorded state. For example, with respect to a data area, the recording is executed by irradiating a laser beam based on, for instance, a blue high power laser onto the whole area.
In the BCA, for example, the BCA marks are formed by irradiating the laser beam only to desired portions in the circumferential direction in correspondence to a pattern of the bar codes corresponding to the content of a disc ID. By the above step, information of a bar code form according to a pattern having bar portions in an un-recorded state of desired widths along the circumferential direction and bar portions in a recording state is recorded.
FIG. 1 shows an example of a waveform of a reproduction signal of the BCA mark of the BD (in the case where the tracking servo control is not made). In the un-recorded state, since a reflectance is high, the reproduction signal of a high level IH is obtained. In the recording state, since the reflectance is low, the reproduction signal of a low level IL is obtained.
However, in the high density optical disc such as a BD in the related art, when the tracking servo control is not made, the reproduction laser beam rides over a plurality of grooves and reads the BCA mark and the signal is modulated by diffraction due to the grooves. Therefore, as shown in FIG. 1, there is such a problem that a fluctuation in level is caused in the reproduction signal of the BCA mark due to a crosstalk of the groove signal and the signal reproduction of the BCA mark becomes unstable.
For example, in the standard of the BD, as a reference for stably reproducing the BCA mark, it has been specified that a ratio IL/IH of the low level IL to the high level IH of the signal obtained after the reproduction signal of the BCA mark passed through an LPF is equal to or less than 0.5. As shown in FIG. 1, when the level fluctuation is large, it is difficult that the ratio between a minimum value IHmin of the high level and a maximum value ILmax of the low level satisfies the value of the standard mentioned above.
It is, therefore, desirable to provide a recording medium which enables binary information which has previously been recorded in a predetermined area to be stably reproduced, a manufacturing method of such recording medium, a mother disc for the recording medium, and a manufacturing method of such a mother disc.